Electronic Design

Long Term Evolution 101

Long Term Evolution is THE fourth generation (4G) cell phone technology that is expected to begin replacing current 3G technologies like cdma2000 and WCDMA/HSPA starting in about 2010.  LTE is a standard being developed by the Third Generation Partnership Project (3GPP).  Final adoption of the standard is expected late in 2009 or early 2010.  This technical system will be submitted to the Internationals Telecommunications Union (ITU) to become a worldwide standard.  The standard is already far enough along so that test equipment manufacturers and base station vendors now have products in place.  LTE is expected to eventually replace most other cellular technologies but the process will be a gradual one as the older technologies will continue to be maintained.  To date, nearly 30 worldwide cellular carriers have committed to LTE.

The impetus behind LTE is the exponential increased use of data services on cell phones including messaging, email, Internet access, music and video downloads.  This has strained the carrier’s infrastructure ability to provide the high speeds expected by subscribers.  LTE will increase data rates and simultaneously boost subscriber capacity while optimizing the use of existing spectrum.

LTE is a mobile broadband system that features high data rates, low latency, improved subscriber capacity and coverage, and IP packet based connectivity.  The maximum data rates are typically 100 Mb/s downlink and 50 Mb/s uplink.  Latency is less than 10 ms.  LTE supports scalable bandwidths of 1.4, 3, 5, 10, 15 and 20 MHz.  Both FDD and TDD duplex modes can be used in either paired or unpaired spectrum. 

The radio technology used in LTE is orthogonal frequency division multiplexing (OFDM).  The access technology is OFDMA.  The OFDM uses that amount of spectrum selected by the carrier or the user’s need and service agreement.  The spectrum is divided up into resource blocks 180 KHz wide.  Each resource block is made up of 12 OFDM subcarriers 15 kHz wide.  The number of resource blocks used is a function of the channel bandwidth selected.  The modulation options are QPSK, 16QAM and 64QAM.

The uplink technology is single carrier frequency division multiple access (SC-FDMA).  This is a technology similar to OFDM and is often called spread OFDM where the OFDM signal is only a single carrier that is further spread by coding over the available bandwidth to improve link reliability from the handset.

An important feature of LTE is the incorporation of multiple input multiple output (MIMO) antenna technology.  MIMO can increase data speeds as well as provide mitigation of multipath issues creating a more reliable connection.  A variety of MIMO formats are supported including 1x2, 2x1, 2x2 and 4x4.  Using a 20 MHz bandwidth a downlink speed of up to 300 Mb/s can be achieved with 4x4 MIMO and up to 150 Mb/s with 2x2 MIMO.

LTE will operate in most cellular spectrum assignments where space is available.  Most LTE is expected to be used in the higher cellular bands between 1800 and 2600 MHz depending upon the country of origin and the carrier’s spectrum assignments.  The new 700 MHz spectrum is also expected to be used for LTE.

With 3G cellular systems still rolling out, it is not expected that LTE will be implemented on a large scale until 2010 and beyond.  As with most new technologies, adoption is gradual as carriers upgrade their systems given OPEX and CAPEX limitations.

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