LTE throughput Calculation

How to calculate LTE peak capacity?

This is the maximum possible capacity which in reality can only be achieved in lab conditions. Let’s assume a 5 MHz LTE system with 2×2 MIMO.

We first calculate the number of resource elements (RE) in a subframe (a subframe is 1 msec):

12 Subcarriers x 7 OFDMA Symbols x 25 Resource Blocks x 2 slots = 4,200 REs

Then we calculate the data rate assuming 64 QAM with no coding (64QAM is the highest modulation for downlink LTE):

6 bits per 64QAM symbol x 4,200 Res / 1 msec = 25.2 Mbps

The MIMO data rate is then 2 x 25.2 = 50.4 Mbps. We now have to subtract the overhead related to control signaling such as PDCCH and PBCH channels, reference & synchronization signals, and coding. These are estimated as follows:

1- PDCCH channel can take 1 to 3 symbols out of 14 in a subframe. Assuming that on average it is 2.5 symbols, the amount of overhead due to PDCCH becomes 2.5/14 = 17.86 %.

2-Downlink RS signal uses 4 symbols in every third subcarrier resulting in 16/336 = 4.76% overhead for 2×2 MIMO configuration.

3-The other channels (PSS, SSS, PBCH, PCFICH, PHICH) added together amount to ~2.6% of overhead.

4-The total approximate overhead for the 5 MHz channel is 17.86% + 4.76% + 2.6% = 25.22%.

5- The peak data rate is then 0.75 x 50.4 Mbps = 37.8 Mbps.

In general, we can say that LTE system bares around 25% gross overhead. Therefore; The following simplified formula can be used for speed calculation:

Example:

Let us assume LTE system with the following configuration

BW: 20MHz                 Modulation: 64QAM            Coding Factor: 1                  MIMO: 2 x 2

Then,

Note that the uplink would have lower throughput because the modulation scheme for most device classes is 16QAM in SISO mode only.

To conclude, the LTE capacity depends on the following:

• Channel bandwidth
• Network loading: number of subscribers in a cell which impacts the overhead
• The configuration & capability of the system: whether it’s 2×2 MIMO, SISO, and the MCS scheme.