LTE Frame Structure

LTE Radio Frames, LTE Slots and LTE Sub frames:

In this topic we will learn  lte frame structure, resource block in lte, lte uplink frame structure, lte tdd frame configuration, lte resource grid, number of samples in lte frame, lte symbol duration

LTE Radio Frames:

Radio Frames in LTE  are of two types.

There are two types of radio frame for LTE: 
1. frame structure is applicable to Frequency Division Duplex (FDD) 
2. frame structure is related to Time Division Duplex (TDD)


1. Frequency Division Duplex (FDD) is valid for both half duplex, and full duplex FDD modes. Type 1 radio frame has a duration 10 ms and consists of equally sized 20 slots each of 0.5 ms. A subframe comprises two slots, thus one radio frame has 10 sub‐frames. In FDD mode, half of the subframes are available for downlink, and the other half are available for uplink transmission in each 10 ms interval, where downlink and uplink transmission are separated in the frequency domain.

2. Time Division Duplex (TDD) radio frame is composed of two identical half‐frames, each one having a duration of 5 ms; each half‐frame is further divided into 5 subframes having a duration of 1ms; two slots of length 0.5 ms constitute a subframe which is not a special subframe. The special types of subframes are composed of three fields: Downlink Pilot Timeslot (DwPTS), GP (Guard Period) and Uplink Pilot Timeslot (UpPTS). Seven uplink-downlink configurations are supported with both types (10 ms and 5 ms) of downlink‐to‐uplink switch‐point periodicity. In 5m downlink‐to‐uplink switch‐point periodicity, special type of sub‐frames is used in both half‐frames, but it is not the case in 10 ms downlink‐to‐uplink switch‐point periodicity, special frame are used only in first half‐frame. For downlink transmission sub‐frames 0, 5 and DwPTS are always reserved. UpPTS and the sub‐frame next to the special sub‐frame are always reserved for uplink communication.

FDD mode bit rate calculation

The basic EUTRAN Radio Frame is 10 ms long.

The EUTRAN Radio Frame is divided into 20 slots, each one 0.5 ms long.

Always two slots together form a sub frame. The sub frame (1 ms) is the smallest time unit the scheduler assigns to physical channels.

In case of FDD there is a time offset between uplink and downlink transmission.

If TDD mode is used, sub frame 0 and sub frame 5 must be downlink, all other sub frames can dynamically be used as uplink or downlink period.

LTE Slot:

• 7 symbols with short cyclic prefix.

• 6 symbols with long prefix.

• EUTRAN combines OFDM symbols in so called resource blocks RB.

• A single resource block is always 12 consecutive subcarriers during one subframe (2 slots, 1 ms):

• 12 subcarriers * 15 kHz= 180 kHz

• It is the task of the scheduler to assign resource blocks to physical channels belonging to different users or for general system tasks.

• A single cell must have at least 6 resource blocks (72 subcarriers) and up to 110 are    possible (1320 subcarriers). 

• OFDM symbols are arranged in a 2 dimensional matrix called the resource grid:

•  –      One axis of the grid is the subcarrier index

•  –      The other axis is the time.

• Each OFDM symbol has its place in the resource grid. 

• Channel estimation based on reference symbols.

• Interpolation in time and frequency domain

• In  WCDMA common pilot channel (CPICH) was used for this (together with reference       symbols on DCH)

• Each OFDM symbol even within a resource block can have a different modulation        scheme.

• EUTRAN defines the following options:  QPSK, 16QAM, 64QAM.

• Not every physical channel will be allowed to use any modulation scheme: Control        channels to be using mainly QPSK.

• From the 3gpp specification:

• In general it is the scheduler that decides which form to use depending on carrier quality feedback information from the UE.

The LTE Slot carries:

OFDM Resource Block for LTE/EUTRAN

OFDM resource Grid for LTE/EUTRAN

OFDM resource Grid for LTE/EUTRAN

Channel Estimation

Modulation Schemes for LTE/EUTRAN

LTE bit rate calculation

       -1 Radio Frame = 10 Sub-frame

     LTE bit rate calculation

       -1 Sub-frame = 2 Time-slots

       -1 Time-slot = 0.5 ms (i.e 1 Sub-frame = 1 ms)

       -1 Time-slot = 7 Modulation Symbols (when normal CP length is used)

       -1 Modulation Symbols = 6 bits; if 64 QAM is used as modulation scheme

    Radio resource is manage in LTE as resource grid....

       -1 Resource Block (RB) = 12 Sub-carriers

    Assume 20 MHz channel bandwidth (100 RBs), normal CP

    Therefore, number of bits in a sub-frame

    = 100RBs x 12 sub-carriers x 2 slots x 7 modulation symbols x 6 bits

    = 100800 bits

   Hence, data rate = 100800 bits / 1 ms = 100.8 Mbps

  * If 4x4 MIMO is used, then the peak data rate would be 4 x 100.8 Mbps = 403 Mbps.

  * If 3/4 coding is used to protect the data, we still get 0.75 x 403 Mbps = 302 Mbps as data      rate.