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.