LTE and DL transmission rate

In the continuation of a recent post on LTE in Moscow, a short explanation of how the width of the spectrum and the “megabits” in LTE are related, and also what DL speed can depend on other than the spectrum width. For example, Downlink, FDD.



The picture shows how the basic resource allocated to the user is formed in the radio interface - Scheduling Block (SB). One SB consists of 14 OFDM symbols in a Time Domain with a total duration of 1 ms (1 TTI - Time Transmission Interval in LTE, unlike, for example, WCDMA, where 1TTI = 10 ms or now 2 ms) and 12 subcarriers in the frequency domain (Frequency Domain), 12 kHz wide.

One OFDM symbol may transmit, depending on the modulation type, 2, 4 or 6 bits of information (QPSK, 16QAM or 64QAM, respectively). Modulation will be selected from those supported by the mobile terminal (UE) depending on the quality of the radio conditions.

Suppose we use 64QAM, 6 bits / symbol, total, during the duration of one sub-frame on one subcarrier, 6 [bits] x 14 [characters] = 84 bits / ms can be transmitted. Further, 12 x 84 [kbit / s] = 1008 kbit / s, well, or one megabit roughly, for one SB. The picture shows how SB can be distributed to users. A spectrum with a width of 20 MHz contains 100 SB (there are actually 110 - 20 MHz / 180 kHz there, but 100, 10 are available for isolation between the bands), so if there is only one user, he can get no more than 1008 kbit / s x 100 = 100.8 Mbps. Or, for example, 6 blocks fit into a spectrum with a width of 1.4 MHz, that is 6.048 Mbit / s in a similar situation.

It’s all for one antenna, without using MIMO (Multiple Input - Multiple Output) modes, which due to the use of spatial multiplexing of data streams (Spatial Multiplexing) allow transmitting not one, as in SISO (Single Input - Single Output), but 2 or more data streams, for example, MIMO 2x2 (2 transmitting antennas on a BS, 2 receiving antennas on a terminal) allows these 100 megabits described above to be increased to 170. The question is which category the terminal is.

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