Evolved Packet System (EPS): The LTE and SAE Evolution of 3G UMTS

3.7 Downlink Scheme: OFDMA (FDD/TDD)

The basic downlink parameters are provided in Table 3.1. Several transmission bandwidths (BW) are possible, in order to allow the positioning of the E-UTRAN physical layer in various spectrum gaps. The value of the narrow band spectrum that modulates each subcarrier is 15 kHz (OFDM symbol duration = 66.67 μs); 7.5-kHz subcarrier spacing, more sensitive to frequency offset, is considered for MBMS-dedicated channels, which gives a larger OFDM symbol duration (133 μs). Granted the constant gap of 15 kHz between subcarriers, the FFT size is growing as a function of selected bandwidth.

Two cyclic prefix lengths are possible, depending on the delay dispersion characteristics of the cell. The longer cyclic prefix (16.67 μs) should then target multi-cell broadcast (MBMS) and very-large-cell scenarios, for instance, for rural and low data rate applications at a price of bandwidth efficiency.

Table 3.1 Parameters for downlink transmission scheme.

In what follows, the MBSFN acronym stands for Mobile Broadcast Single Frequency Network and refers to mobile broadcasting network using a single band on which dedicated and broadcasted signals are sharing a single ‘carrier’. This notion will be developed in Section 3.8.11.

The short CP (5.21 μs) accommodates less delay spread and is suitable for urban and high data rate application. The number of OFDM symbols ...