LTE Throughput Analysis – Part 1

LTE Downlink Throughput Calculation Example

As follow you can find an example of TP Calculation.

Let’s start to assume:

• • • 20 MHz channel bandwidth
    • Normal Cycling Prefix (CP)
    • 4×4 MIMO

Before to start let’s remind some terms used in an LTE Frame.

There are six time units: frame, half-frame, subframe, slot, symbol, and the basic time unit (Ts), as shown in the following table.

Below is an illustration of an FDD frame.

I order to be able to calculate the right Throughput in Downlink we will have to:

1. 1. Calculate the number of Resource Elements (RE or subcarriers) in a subframe with 20 MHz channel bandwidth. So, 12 subcarriers x 7 OFDMA symbols x 100 RB x 2 slots= 16800 REs per subframe.
      Each RE can carry a modulation symbol so we will have 16,8 Msps
   2. Assuming a 64QAM modulation with no coding we will have that one modulation symbol will carry on 6 bits and the total bits in a subframe (1ms) over 20 MHz channel will be 16800 modulation symbols x 6 bits / modulation symbol = 100800 bits/1 ms 
      Than, finally the data rate is 100.8 Mbps (See Table Below).
   3. With a 4×4 MIMO, the peak data rate goes up to 100.8 Mbps x 4 = 403 Mbps
   4. Estimating about 25% overhead such as PDCCH, Reference Signal, Sync Signals, PBCH, and some coding. We get 403 Mbps x 0.75 = 302 Mbps

Table 1: Absolute Physical Throughput with all Elements Allocated to the PDSCH

The throughputs within Table 1 is generated by multiplying the modulation symbol rate by the number of bits per symbol.

For example, the 20 MHz channel bandwidth has 100 RB (Resource Blocks) providing 1200 subcarriers in the frequency domain. When using the normal cyclic prefix there are 14 OFDMA symbols during each 1 ms subframe so the modulation symbol rate is given by 1200  14 / 0.001 = 16.8 Msps.

The bit rate when using 64QAM is then given by 16.8 Msps x 6 bits per symbol = 100.8 Mbps

Absolute Maximum Physical Throughput with NORMAL CYCLING PREFIX

Absolute Maximum Physical Throughput with EXTENDED CYCLING PREFIX

To calculate it more accurately we must to check the 3GPP specs 36.213, table 7.1.7.1-1 and table 7.1.7.2.1-1

Table 7.1.7.1-1: Modulation and TBS index table for PDSCH (3GPP TX 36.213)

Table 7.1.7.1-1 shows the mapping between MCS (Modulation and Coding Scheme) index and TBS (Transport Block Size) index.

Let’s pick the highest MCS index 28 (64QAM with the least coding), which is mapping to TBS index of 26.

Table 7.1.7.2.1-1: Transport block size table (3GPP TS 36.213)

Table 7.1.7.2.1-1 shows the transport block size. It indicates the number of bits that can be transmitted in a subframe/TTI (Transmit Time Interval).

For example, with 100 RBs and TBS index of 26, the TBS is 75376.

Assuming a 4×4 MIMO, the peak data rate will be 75376 x 4 = 301.5 Mbps.