DFTS-OFDM, fast facts

DFTS-OFDM, fast facts

DFTS-OFDM is used in the LTE uplink

Discrete Fourier Transform Spread OFDM (DFTS-OFDM) is used for the LTE uplink.

Basic properties

• Also known as Single Carrier Frequency Division Multiple Access (SC-FDMA)
• Small variations in transmitted power (and low PAR).
• Possible to use low-complexity, highly efficient equalization in a frequency selective channel.
• Possibility to use FDMA with flexible bandwidth assignment.

·         Same as for the downlink, the sub-carrier spacing is 15 khz and a resource block consists of 12 sub-carriers

Basic principles

• DFTS-OFDM can be interpreted as normal OFDM with DFT-based pre-coding.
• Main benefit of DFTS-OFDM is the reduced variations in transmitted power (and low PAR).
• An equalizer is needed to compensate for the radio channel frequency selectivity (MMSE).

·         Similar to OFDM, the physical resource is a time-frequency grid of resource elements/blocks.

·         Un-like conventional OFDM. The sub-carriers must be adjacent in frequency.

Uplink reference signals

·         Demodulation reference signals (DRS) is needed for channel estimation and coherent demodulation.

·         DRS are always transmitted together with the corresponding physical channel to be demodulated (PUSCH or PUCCH)

·         Sounding Reference Signals (SRS) are transmitted for estimation of the channel quality at different frequencies.

·         SRS are used as basis for assigning resource blocks to a user.

Uplink L1/L2 control signaling

·         Main content is: HARQ ack/nack's, reports on downlink channel conditions and scheduling requests.

·         Unlike the downlink, there is no need to indicate the transport format since the terminal always uses the transport format details that were sent in the scheduling grants by the NW.

·         The basis for channel status reports is aperiodic reports where the enodeB requests a report by setting the CQI request bit in a scheduling grant

Uplink scheduling

• In EUL, the scheduling grant consists of a Tx power limit that the terminal is not allowed to exceed. If a terminal does not exploit all if it’s granted Tx power, it is available for use by another terminal.
• LTE UL scheduling grants consists of resource blocks (time, frequency) which the terminal will always use for data or fill up with padding bits.
• The e nodeB decides which transport format (TBS, modulation, code rate) the terminal must use.
• Since the e nodeB knows the transport format and resource blocks, there is no need for outband signaling of this information.

Channel status reporting

The channel status reports consists of one or several pieces of information

• Rank Indication (RI)
• Pre-coder Matrix Indication (PMI)
• Channel Quality Indication (CQI)

Channel status reports can be

• Wideband (one report) or sub-band (several reports)
• Periodic on a fixed interval and a fixed UL scheduling grant
• A-periodic or trigger-based, following a request from the NW or a fulfilled event in the terminal.
• If padding is applied and the number of padding bits exceeds the number required for a channel status report. A report will then be sent instead.