Monday, February 27, 2012

4G LTE air interface - Radio Frequency Aspects

LTE air interface adopts OFDMA (orthogonal frequency division multiple access) for the downlink and SC-FDMA (single-carrier frequency domain multiple access) for the uplink. It employs AMC (adaptive modulation and coding) as well as a number of optional MIMO (multiple-input multiple-output) antenna techniques and interference management methods for potential performance enhancements.

Telecommunications Blog

One of the first things to note about LTE is the integration between the Frequency Division Duplex (FDD) and Time Division Duplex (TDD) radio access modes. In the previous Universal Mobile Telephone System (UMTS) specifications, which also supported FDD and TDD, the RF specifications for the UE FDD, UE TDD, base station FDD and base station TDD modes were covered in separate documents.

However, the early decision by 3GPP to fully integrate FDD and TDD modes for LTE has resulted in only one RF specification document each for the UE and the eNB. With the higher level of integration between the two modes, the effort required to support them should be less than it was in the past.

Channel Bandwidths:

LTE was designed from the start to support six different channel bandwidths. These are 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. In addition to this the subcarriers are spaced 15 kHz apart from each other. To maintain orthogonality, this gives a symbol rate of 1 / 15 kHz = of 66.7 ยตs.

Earlier versions of the specifications also supported 1.6 MHz and 3.2 MHz for interworking with LCR TDD, but these were removed when the LTE TDD frame structure was aligned with the FDD frame structure rather than the TD-SCDMA frame structure from UMTS.

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