LTE Radio Frames

LTE Radio Frame:                                          Reference: 3G

• Air Interface For 3G LTE.
• Manage the different types of information that needs to be carried between the eNodeB and the User Equipment.
• The frame structures for LTE differ between the Time Division Duplex, TDD and the Frequency Division Duplex, FDD
• Two adjacent slots constitute a sub-frame of length 1 ms
• There are two types of LTE frame structure:

• Type 1:   used for the LTE FDD mode systems.
• Type 2:   used for the LTE TDD systems.

Type 1 LTE Frame Structure

• The basic type 1 LTE frame has an overall length of 10 ms.
• This is then divided into a total of 20 individual slots. 
• LTE Sub-frames then consist of two slots - in other words there are ten LTE sub-frames within a frame.

Type 1 LTE Frame Structure

Type 2 LTE Frame Structure

• The frame structure for the type 2 frames used on LTE TDD.
• The 10 ms frame comprises two half frames, each 5 ms long.



Type 2 LTE Frame Structure

• The LTE half-frames are further split into five sub-frames, each 1ms long.

• The sub-frames may be divided into
• Standard sub-frames or
• Special sub-frames.

The special sub-frames consist of three fields;

• DwPTS - Downlink Pilot Time Slot
• GP - Guard Period
• UpPTS - Uplink Pilot Time Stot.

The fields are individually configurable in terms of length, although the total length of all three together must be 1ms.

LTE TDD sub-frame positions:

• Possible to dynamically change the up and down-link balance.
• Characteristics to meet the load conditions.
• A total of seven up / down-link configurations have been set.
• These use either 5 ms or 10 ms switch periodicity.
• In the case of the 5ms switch point periodicity, a special sub-frame exists in both half frames.
• In the case of the 10 ms periodicity, the special sub-frame exists in the first half frame only.
• Sub-frames 0 and 5 as well as DwPTS are always reserved for the down-link.
• UpPTS and the sub-frame immediately following the special sub-frame are always reserved for the up-link transmission.

Uplink-downlink configuration	Downlink to uplink switch periodicity	Subframe number
Slot No		0	1	2	3	4	5	6	7	8	9
0	5 ms	D	S	U	U	U	D	S	U	U	U
1	5 ms	D	S	U	U	D	D	S	U	U	D
2	5 ms	D	S	U	D	D	D	S	U	D	D
3	10 ms	D	S	U	U	U	D	D	D	D	D
4	10 ms	D	S	U	U	D	D	D	D	D	D
5	10 ms	D	S	U	D	D	D	D	D	D	D
6	5 ms	D	S	U	U	U	D	S	U	U	D

Where:
    D is a sub-frame for down-link transmission
    S is a "special" sub-frame used for a guard time
    U is a sub-frame for up-link transmission

Uplink / Downlink subframe configurations for LTE TDD

INTRODUCTION TO LTE

INTRODUCTION TO LTE:                                          Reference: 3GPP, Agilent app note

Ø  Third-generation UMTS, has been deployed all over the world.

Ø  To ensure that this system remains competitive in the future, in November 2004 3GPP began a project to define the long-term evolution of UMTS cellular technology.

Ø  The specifications related to this effort are formally known as the evolved UMTS terrestrial radio access (E-UTRA) and evolved UMTS terrestrial radio access network (E-UTRAN), but are more commonly referred to by the project name LTE.

Ø  The first version of LTE is documented in Release 8 of the 3GPP specifications.

Ø  A parallel 3GPP project called System Architecture Evolution (SAE) is defining a new all-IP, packet-only core network (CN) known as the evolved packet core (EPC).

Ø  The combination of the EPC and the evolved RAN (E-UTRA plus E-UTRAN) is the evolved packet system (EPS).

Summary of LTE requirements:

Ø  Increased downlink and uplink peak data rates.

Ø  Scalable channel bandwidths of 1.4, 3, 5, 10, 15, and 20 MHz in both the

Ø  uplink and the downlink.

Ø  Spectral efficiency improvements over Release 6 high speed packet access (HSPA) of three to four times in the downlink and two to three times in the uplink

Ø  Sub-5 ms latency for small internet protocol (IP) packets

Ø  Performance optimized for low mobile speeds from 0 to 15 km/h.

Ø  Supported with high performance from 15 to 120 km/h.

Ø  Functional support from 120 to 350 km/h, under consideration for 350 to 500 km/h

Ø  Co-existence with legacy standards while evolving toward an all-IP network

LTE Requirements

Multiple access technology:

Downlink and uplink transmission in LTE are based on the use of multiple access technologies: specifically,

Ø  Downlink : Orthogonal frequency division multiple access (OFDMA)

Ø  Uplink: Single-carrier frequency division multiple access (SC-FDMA) for the uplink.

OFDMA:

Ø  OFDMA is a variant of orthogonal frequency division multiplexing (OFDM).

Ø  Rather than transmit a high-rate stream of data with a single carrier, OFDM makes use of a large

Ø  number of closely spaced orthogonal subcarriers that are transmitted in parallel.

Ø  Each subcarrier is modulated with a conventional modulation scheme (such as QPSK, 16QAM, or 64QAM) at a low symbol rate.

Ø  The combination of hundreds or thousands of subcarriers enables data rates similar to conventional single-carrier modulation schemes in the same bandwidth.

Ø  In the frequency domain, multiple adjacent subcarriers are each independently modulated with data.

Ø  Then in the time domain, guard intervals are inserted between each of the symbols

to prevent inter-symbol interference at the receiver.

SC-FDMA:

Ø  The high peak-to-average ratio (PAR) associated with OFDM led 3GPP to look for a different transmission scheme for the LTE uplink.

Ø  SC-FDMA was chosen because it combines the low PAR techniques of single-carrier transmission systems.

Ø  Data symbols in the time domain are converted to the frequency domain using a discrete Fourier transform (DFT).

Ø  In the frequency domain they are mapped to the desired location in the overall channel bandwidth before being converted back to the time domain using an inverse FFT (IFFT).

Ø  Finally, the CP is inserted.