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Wednesday 18 January 2012

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.















2 comments:

  1. hi, i have a question..
    why SCFDMA is used for uplink and OFDMA for the downlink?

    ReplyDelete
    Replies
    1. Because of PAPR. To reduce the battery consumption, In Uplink Single carrier FDMA is used..
      For more on SCFDMA go to this Video @ http://www.youtube.com/watch?v=tLEw50Fytxc

      Delete