LTE AIR INTERFACE - PHYSICAL LAYER

The LTE physical layer is based on Orthogonal Frequency Division Multiplexing scheme OFDM to meet the targets of high data rate and improved spectral efficiency. The spectral resources are allocated/used as a combination of both time (aka slot) and frequency units (aka subcarrier). MIMO options with 2 or 4 Antennas is supported. Multi-user MIMO is supported in both UL and DL. It also depends upon UE capabilites.The modulation schemes supported in the downlink and uplink are QPSK, 16QAM and 64QAM.

Downlink (DL) Physical Channel

The downlink transmission uses the OFDM with cyclic prefix . Some of the reasons for using OFDM are given below:

• Multiple carrier modulation (MCM) helps in countering the frequency selective fading as the channel appears to have nearly flat frequency response for the narrow band subcarrier.
• The frequency range of the resource block and the number of resource blocks can be changed (or adapted to the channel condition) allowing flexible spectrum allocation.
• Higher peak data rates can be achieved by using multiple resource blocks and not by reducing the symbol duration or using still higher order modulation thereby reducing the receiver complexity.
• The multiple orthogonal subcarriers inherently provides higher spectral efficiency.
• The cyclic prefix (CP) is the partial repetition of the bit/symbol sequence from the end to the beginning. This makes the time domain input sequence to appear periodic over a duration so that the DFT representation is possible for any frequency domain processing. Also the duration if chosen larger than the channel delay spread, will help in reducing the inter-symbol interference.

The following pilot signals are defined for the downlink physical layer:

• Reference signal: The reference signal consists of known symbols transmitted at a well defined OFDM symbol position in the slot. This assists the receiver at the user terminal in estimating the channel impulse response so that channel distortion in the received signal can be compensated for. There is one reference signal transmitted per downlink antenna port and an exclusive symbol position is assigned for an antenna port (when one antenna port transmits a reference signal other ports are silent).
• Synchronization signal: Primary and secondary synchronization signals are transmitted at a fixed subframes (first and sixth) position in a frame and assists in the cell search and synchronization process at the user terminal. Each cell is assigned unique Primary sync signal.

Uplink (UL) Physical Channel

The uplink transmission uses the SC-FDMA (Single Carrrier FDMA) scheme. The SC-FDMA scheme is realized as a two stage process where the first stage transforms the input signal to frequency domain (represented by DFT coefficients) and the second stage converts these DFT coefficients to an OFDM signal using the OFDM scheme. Because of this association with OFDM, the SC-FDMA is also called as DFT-Spread OFDM. The reasons (in addition to those applicable for OFDM for downlink) for this choice are given below:

• The two stage process allows selection of appropriate frequency range for the subcarriers while mapping the set of DFT coefficients to the Resource Blocks. Unique frequency can be allocated to different users at any given time so that there is no co-channel interference between users in the same cell. Also channels with significant co-channel interference can be avoided.
• The transformation is equivalent to shift in the center frequency of the single carrier input signal. The subcarriers do not combine in random phases to cause large variation in the instantaneous power of the modulated signal. This means lower PAPR (Peak to Average Power Ratio).
• The PAPR (Peak to Average Power Ratio) of SC-FDMA is lesser than that of the conventional OFDMA, so the RF power amplifier (PA) can be operated at a point nearer to recommended operating point. This increases the efficiency of a PA thereby reducing the power consumption at the user terminal.

The following pilot signals are defined for the uplink physical layer:

• Demodulation Reference signal: This signal send by the user terminal along with the uplink transmission, assists the network in estimating the channel impulse response for the uplink bursts so as to effectively demodulate the uplink channel.
• Sounding Reference Signal: This signal is sent by the user terminal assists the network in estimating the overall channel conditions and to allocate appropriate frequency resources for uplink transmission.