OFDM fast facts

OFDM fast facts

OFDM is used for the LTE downlink

Basic principles

• OFDM is highly robust to channel frequency selectivity due to the relatively long symbol period in combination with a cyclic prefix
• OFDM enables scheduling in the frequency domain as well as the time domain.
• Large number of frequency adjacent sub-carriers which are used for flexible transmission bandwidth by simply changing the number of subcarrier that are allocated at each scheduling moment (per TTI).
• Rectangular pulse shaping corresponding to sinc-square shaped per-sub-carrier spectrum.
• Tight sub-carrier spacing, in LTE 15khz.
• During each OFDM symbol interval, multiple modulation symbols are transmitted in parallel on the sub-carriers.
• QPSK, 16QAM, 64QAM can be used.
• Sub-carrier orthogonality is made possible due to the sinc-square shaped sub-carrier spectrum combined with a sub-carrier spacing equal to the per sub-carrier symbol rate
• Fast Fourier Transform (FFT) processing

Frequency diversity with OFDM

• Channel coding is used to spread the information bits over many code bits. These code bits are then mapped, via the OFDM symbols, over a set of sub-carriers that are spread over the overall OFDM transmission bandwidth spectrum. This distribution of the code bits in the frequency domain is known as frequency interleaving which is similar to time domain interleaving.
• LTE makes use of interleaving in both the frequency domain and the time domain as compared to WCDMA/HSPA which only uses the time domain for interleaving.
• The benefits of interleaving are equal in the two domains.

Downlink L1/L2 control signaling

• Downlink L1/L2 control signaling is needed for data transfer in both UL&DL.

• For the downlink it consists of scheduling information necessary for receiving, demodulating and decoding the DL-SCH.
• For the up-link it consists of scheduling grants with information about the resources and transport format to use for UL-SCH. Also, it contains HARQ ACK/NACK’s.

• Downlink L1/L2 control signaling is carried on three different physical channels:

1. Physical Control Format Indicator Channel (PCFICH) – carries information about the size of the control region which is the part of the sub-frame in which the control signaling is transmitted.
2. Physical Downlink Control Channel (PDCCH) – carries downlink scheduling assignments and uplink scheduling grants.

      3    Physical HARQ Indicator Channel (PHICH) carries HARQ ACK/NACK’s

Overall Time Domain Structure

• A radio frame of length 10ms consists of ten equally sized sub-frames of length 1ms.
• Each frame is identified by its System Frame Number (SFN).
• A sub-frame of length 1ms consists of two slots of length 0,5 ms.
• A slot then consists of six or seven symbols including cyclic prefix.
• A resources block consists of 12 sub-carriers during one 0,5ms slot.
• A resources element consists of one sub-carrier during one symbol interval.